Glass sheet designed to be heat tempered

ABSTRACT

The subject of the invention is a glass sheet intended to be thermally toughened, the matrix of which is of the silica-soda-lime type, having an expansion coefficient α of greater than 100×10 −7  K −1 , a Young&#39;s modulus E of greater than 60 GPa and a thermal conductivity k of less than 0.9 W/m.K.

BACKGROUND OF THE INVENTION

The invention relates to glass sheets intended to be thermally toughenedand more precisely to glass sheets intended to be fitted into motorvehicles.

Although the invention is not limited to such applications, it will bemore particularly described with reference to the production ofthermally toughened thin glass sheets, i.e. those having a thickness ofless than 2.5 mm. This is because motor-vehicle manufacturers are at thepresent time increasingly tending to wish to limit the weightcorresponding to the glazing, while the glass area of the motor vehiclesis increasing. A reduction in the thickness of the glass sheets istherefore needed in order to meet these new requirements.

With regard to the thermal toughening of these glass sheets, and moreparticularly in order to produce the side windowpanes of motor vehicles,the requirements of European Regulation No. 43, relating to thehomologation of safety glazing and of the materials for glazing intendedto be fitted into motor vehicles and their trailers, have to be met.According to this regulation, the constraints on toughening must be suchthat the glazing, in the event of it breaking, does so into a number offragments which, over any 5×5 cm square, is neither less than 40 norgreater than 350 (the latter number being increased to 400 in the caseof glazing having a thickness of less than or equal to 2.5 mm). Againaccording to these requirements, no fragment must be greater than 3.5cm², except possibly in a strip 2 cm in width around the periphery ofthe glazing and within a 7.5 cm radius around the point of impact, andthere must not be any elongate fragment of greater than 7.5 cm.

Conventional toughening plants, especially the devices for bending andtoughening glass sheets, by making them run along a roller conveyorhaving a profile that is curved in the direction in which the glasssheet run, allow 3.2 mm thick glass sheets to be toughened according toEuropean Regulation No. 43 completely satisfactorily.

The abovementioned techniques are known, especially from French PatentsFR-B-2,242,219 and FR-B-2,549,465 and consist in making the glasssheets, heated in a horizontal furnace, run between two layers ofrollers —or other rotating elements —arranged with a curvilinear profileand passing through a terminal toughening zone. In order to produce sidewindowpanes, sunroofs or other glazing articles, especially ofcylindrical shape, the layers consist of, for example, right cylindricalrods arranged with a circular profile. The layers may also consist ofelements giving the glazing a secondary curvature, such as conicalelements or else those of the diabolo type or barrel type. Thistechnique allows a very high production capacity since, on the one hand,the glass sheets do not have to be widely spaced, it being possible forone glass sheet to enter the forming zone without any problem while thetreatment of the previous sheet has yet to be completed and, on theother hand, if the length of the rollers so allow, two or three glasssheets side by side may be treated simultaneously.

The running speed of the glass plates or sheets is at least 10 cm/s andis about 15 to 25 cm/s. The speed normally does not exceed 30 cm/s inorder to allow sufficient toughening time.

When the thickness of the glass sheets decreases, and in order to meetthe same toughening standards, the heat-exchange coefficient must begreatly increased. To do this, it is possible to increase the blowingpower of the toughening devices. Such modifications entail, on the onehand, major investment and, on the other hand, higher operating costs.Moreover, the increase in the blowing power may impair the opticalquality of the glass sheets and/or their flatness.

The inventors were thus tasked with the mission of producing glasssheets toughened according to European Regulation No. 43, having athickness of less than 2.5 mm on standard toughening plants of the typedescribed above.

SUMMARY OF THE INVENTION

Thus, the object of the invention is to provide a glass sheet intendedto be thermally toughened, the intrinsic properties of which lead toresults in the case of thicknesses of less than 2.5 mm but areequivalent to those usually obtained in the case of thicknesses ofgreater than 3 mm, with the same cooling devices.

This object is achieved by a glass sheet intended to be thermallytoughened, the matrix of which is of the silica-soda-lime type and hasan expansion coefficient α of greater than 100×10⁻⁷K⁻¹, a Young'smodulus E of greater than 60 GPa and a thermal conductivity k of lessthan 0.9 W/m.K.

Such properties actually give the glass sheet the possibility of beingthermally toughened according to European Regulation No. 43 when thissheet has a thickness of less than 2.5 mm.

DETAILED DESCRIPTION OF THE INVENTION

According to a preferred embodiment of the invention, the glass sheethas a Poisson's ratio of greater than 0.21.

The elastic modulus and the Poisson's ratio are determined by thefollowing test: a glass test piece having the dimensions 100×10 mm² anda thickness of less than 6 mm is subjected to a 4-point bending, theouter supports of which are separated by 90 mm and the inner supports by30 mm. A strain gauge is adhesively bonded to the centre of the glassplate. The principal displacements (in the length of the plate and inits width) are calculated therefrom. The applied stress is calculatedfrom the applied force. The relationships between stress and principaldisplacements allow the elastic modulus and the Poisson's ratio to bedetermined.

Also preferably, the specific heat of the glass sheet is greater than740 J/kg.K.

According to an advantageous embodiment of the invention the glass sheethas a density of greater than 2520 kg/cm³ and preferably greater than2550 kg/m³.

Again preferably, the glass sheet according to the invention satisfiesthe following relationship:

α.E/K>8000

The glass matrices of the glass sheets according to the invention areadvantageously chosen from among matrices having, in percentages byweight, the following constitutes:

Si0₂ 45-69%  Al₂O₃ 0-14% CaO 0-22% MgO 0-10% Na₂O 6-24% K₂O 0-10% BaO0-12% B₂O₃ 0-6%  ZnO  0-10%.

The glass compositions proposed above have the advantage in particularof being able to be melted and converted into glass ribbon on float-typeplants, at temperatures close to those adopted for the manufacture ofconventional silica-soda-lime glass.

The compositions are actually chosen to have a temperature correspondingto the viscosity η, expressed in poise, such that the logη=2 is lessthan 1500° C. in order to allow melting under standard conditions.Moreover, the compositions according to the invention have a sufficientdifference between the forming temperature of the glass and its liquidustemperature; this is because, in the technology of float glass inparticular, it is important for the liquidus temperature of the glass toremain equal to or less than the temperature corresponding to theviscosity, expressed in poise, such that logη=3.5. Advantageously, thisdifference is at least from 10° C. to 30° C.

The SiO₂ content must not exceed 69%; above this, the melting of thebatch and the refining of the glass require high temperatures whichcause the furnace refractories to undergo accelerated wear. Below 45%,the glasses according to the invention are insufficiently stable.Advantageously, the SiO₂ content is greater than 53%.

Alumina acts as a stabilizer; this oxide helps to increase thestrain-point temperature. The Al₂O₃ content must not exceed 14%, or elsemelting becomes too difficult and the high-temperature viscosity of theglass increases unacceptably.

The glass compositions according to the invention may also include theoxide B₂O₃. In this case, the B₂O₃ content does not exceed 6% as, abovethis value, the volatilization of the boron in the presence of alkalimetal oxides during production of the glass may become significant andlead to corrosion of the refractories. Furthermore, higher B₂O₃ contentsimpair the quality of the glass. When B₂O₃ is present in the glasscomposition with a content of greater than 4%, the Al₂O₃ content isadvantageously greater than 10%.

The influence of the other oxides on the ability of the glassesaccording to the invention to be melted and floated on a metal bath, aswell as on their properties, is as follows: the alkali metal oxides, andmore particularly Na₂O and K₂O, make it possible to keep the meltingpoint of the glasses according to the invention and theirhigh-temperature viscosity within acceptable limits. To do this, the sumof the contents of these alkali metal oxides remains greater than 11%and preferably greater than 13%.

The alkaline-earth metal oxides introduced into the glasses according tothe invention also have the effect of reducing the melting point, aswell as the high-temperature viscosity of the glasses. The sum of thecontents of these oxides is at most 6% and preferably greater than 8%.Above approximately 28%, the tendency of the glasses to devitrify mayincrease to levels incompatible with the process of floating them on ametal bath.

The glass compositions may furthermore contain colorants, especially forapplications of the motor-vehicle window type; these may especially beoxides of iron, of chronium, of cobalt, of nickel, of selenium, etc.

According to a first embodiment of the invention, the glass sheetaccording to the invention is such that its matrix comprises, inpercentages by weight, the abovementioned constitutes and satisfies therelationships:

Na₂O+K₂O>20%

Na₂O+K₂O+CaO>27%.

According to a second embodiment of the invention, the glass matrixsatisfies the relationships:

Na₂O+K₂O>17%

Na₂O+K₂O+CaO>35%.

According to other embodiments according to the invention, the glassmatrix satisfies the relationships:

Na₂O+K₂O>17%

Na₂O+K₂O+CaO>29% when Na₂O>18%

and/or K₂O>5%

and/or Al₂O₃<3%.

According to the latter embodiments and when the oxide TiO₂ is presentin the matrix, the latter furthermore satisfies the relationship:

TiO₂+Al₂O₃<3%.

All the glass matrices described according to these various embodimentsallow the production of glass sheets having a thickness of less than 2.5mm and advantageously greater than 1.6 mm, which may be thermallytoughened in accordance with European Regulation No. 43 on tougheningdevices originally intended for toughening glass with a thickness of3.15 mm.

The advantages afforded by the glass compositions according to theinvention will be better appreciated with the help of the examplespresented below.

Various glass compositions in accordance with the invention were meltedand converted into a glass ribbon according to the invention. There are6 of these compositions (numbered from 1 to 6 ). The composition T is acontrol composition, corresponding to standard glass for motor-vehiclewindows, which may be thermally toughened in accordance with EuropeanRegulation No. 43 when it is in the form of a glass sheet with athickness of 3.15 mm.

These various compositions are given in the table below:

TABLE 1 SiO₂ Al₂O₃ CaO MgO Na₂O K₂O BaO T 71.3 0.6 9.6 4.1 13.6 0.3 0 167.1 2.1 8.5 0.1 16.0 5.1 0.1 2 63.59 0.45 13.19 0.07 21.75 0.01 0 363.22 2.45 13.40 0.1 17.5 2.65 0 4 64.8 2.0 10.4 0.5 17.4 4.9 0 5 64.02.0 10.4 0.5 16.3 4.8 2 6 65.0 1.0 14.1 0 18.9 1.0 0

The various properties of the glasses indicated above are given in thefollowing table:

TABLE 2 T 1 2 3 4 5 6 Expansion 90 11 12 12 12 12 12 coefficient (10⁻⁷K⁻¹) 6 8 0 2 0 0 Young's modulus 70 68 70 70 68 68 69 (GPa) Thermal 10.8 0.8 0.8 0.8 0.8 0.8 Conductivity (W/m.K) 5 7 3 6 3 5 Specific heat85 85 87 85 85 84 86 (J/kg.K) 5 2 2 7 7 3 1 Density 25 25 26 26 26 27 26(kg/m³) 80 60 60 48 26 31 70 Poisson's 0.2 0.2 0.2 0.2 0.2 0.2 0.2 ratio2 2 3 3 3 3 3

It has been demonstrated that these glasses can be melted and that theycan be for the most part converted using the float process.

When testing these glass compositions, it is apparent that they can bemelted under completely conventional conditions and even at temperatureswell below those of the control composition T. These temperaturedifferences make it possible to envisage a reduction in energy costs.

On the other hand, if it appears that the forming ranges, i.e. thedifferences between the temperature corresponding to a viscosity η,expressed in poise, such that logη=3.5 and the liquidus temperature, arenarrower in the case of the compositions according to the invention;however, they are sufficient to guarantee good-quality forming.

It is also apparent that the initial toughening temperature is markedlylower in the case of the glasses according to the invention; this alsoleads to energy cost reductions and to less rapid water of the furnaces.

The final table given below shows the thickness of the glass sheetswhich have been toughened in accordance with European Regulation No. 43.

TABLE 3 T 1 2 3 4 5 6 Thickness (mm) 3.1 2.5 2.4 2.3 2.4 2.4 2.4 5 0 0 50 5 5

It is therefore clearly apparent that the glass sheets produced from thecompositions according to the invention allow so-called “safety” thermaltoughening for thicknesses of less than 2.5 mm using the standarddevices which limit the said toughening to a thickness of 3.15 mm whenthe glass is of the composition T.

Moreover, the optical quality of the glass sheets according to theinvention, having a thickness of less than 2.5 mm and thermallytoughened, is quite comparable to that of glass sheets having athickness of 3.15 mm produced from the control composition T.

What is claimed is:
 1. A glass sheet intended to be thermally toughened comprising a silica-soda matrix, wherein said sheet has an expansion coefficient of greater than 100×10⁻⁷K⁻¹, a Young's modulus E of greater than 60 GPa and a thermal conductivity k of less than 0.9 W/m.K and said matrix has a SiO₂ content of 45 to 65 wt. %, wherein said matrix comprises Na₂O and optionally K₂O in amounts which satisfy the following relationship: Na₂O+K₂O>20 wt % and wherein said matrix comprises Na₂O and CaO and optionally K₂O in amounts which satisfy the following relationship: Na₂O+K₂O+CaO>27 wt %.
 2. A glass sheet intended to be thermally toughened, comprising a silica-soda matrix, wherein said sheet has an expansion coefficient a of greater than 100×10⁻⁷K⁻¹, a Young's modulus E of greater than 60 GPa and a thermal conductivity k of less than 0.9 W/m.K and said matrix has a SiO₂ content of 45 to 65 wt %, wherein said matrix comprises Na₂O and optionally one or more of K₂O or CaO in amounts which satisfy the following relationship: Na₂O+K₂O+CaO>27 wt %.
 3. A glass sheet intended to be thermally toughened, comprising a silica-soda matrix, wherein said sheet has an expansion coefficient a of greater than 100×10⁻⁷K⁻¹, a Young's modulus E of greater than 60 GPa and a thermal conductivity k of less than 0.9 W/m.K and said matrix has a SiO₂ content of 45 to 65 wt %, wherein said matrix comprises Na₂O and CaO and optionally K₂O in amounts which satisfy the following relationships: Na₂O+K₂O>17% Na₂O+K₂O+CaO>35 wt %.
 4. A glass sheet comprising a silica-soda matrix, wherein the sheet has an expansion coefficient a of greater than 100×10⁻⁷K⁻¹, a Young's modulus E of greater than 60 Gpa and a thermal conductivity k of less than 0.9 W/m.K, and wherein the matrix comprises Na₂O and optionally one or more of K₂O or CaO in amounts that satisfy the following relationship: Na₂O+K₂O+CaO>35 wt %.
 5. Glass sheet according to claim 4, wherein said matrix satisfies the relationship: Na₂O+K₂O>17 wt %. 